The present invention relates generally to synthetic array radar (SAR) autofocus processing techniques, and more particularly, to a phase difference autofocus method that is adapted to compensate for quadratic and cubic phase errors.
SAR resolution and the utility of its imagery heavily depends upon an ability to accurately estimate a residual phase error from the range compressed video phase history (VPH) data of a SAR data. In a typical autofocus method, a residual phase error is assumed to be accurately representable by a polynomial, and the autofocus processing is normally designed to estimate the coefficients of that polynomial. Many autofocus methods, however, have a limited pull-in range. The pull-in range is that point wherein the ability to estimate phase error begins to break down if the amount of a phase error becomes too excessive.
With reference to U.S. patent application Ser. No. 07/798,783, filed Nov. 27, 1991, for "Multiple Discrete Autofocus," assigned to the assignee of the present invention, it has been found that it is highly desirable to estimate and compensate for quadratic and cubic phase errors from the range compressed VPH data before applying a multiple discrete autofocus method. In the multiple discrete autofocus method, residual quadratic phase error is removed from the video phase history data before it is bandpass filtered. This is normally achieved by the use of a phase difference autofocus method that has been shown to be very effective in estimating the amount of quadratic phase error.
One method that has a virtually unlimited pull-in range for a quadratic phase error is a phase difference autofocus method. This method that is described in U.S. Pat. No. 4,999,635, for "Phase Difference Autofocusing for Synthetic Aperture Radar Imaging," assigned to the assignee of the present invention. This method has been shown to be very effective in accurately estimating the amount of residual quadratic phase error in range compressed VPH data. This high performance is primarily due to the fact that any quadratic phase error resident in a full array is essentially cancelled out when two subarrays are multiplied to form a second-order-product subarray.
For quadratic phase error estimation, at least two subarrays are required to form at least one second-order-product subarray. For a cubic phase error estimation, at least three subarrays are needed to form two second-order-product subarrays. The implementation of the second-order-product phase difference autofocus method is described in the above-cited patent application.
However, it has been determined that the performance of phase error estimation degrades as the amount of cubic phase error in a full array increases. This is caused by the fact that second-order-product subarrays still contain quadratic phase errors, which adversely affect the ability to detect the location of the proper peak in FFT filter processing. In such a situation, more than one autofocus iteration is normally required to accurately estimate the amount of phase errors.